1. Field of the Invention
The present invention relates to a magnetic recording and playback apparatus which records and reproduces text, graphics and other signals superposed in the vertical blanking period of a television signal (hereinafter, these signals will be referred to as the so-called "teletext" signals).
2. Description of Prior Arts
Various recording and playback systems have been proposed for home video tape recorders (hereinafter referred to as VTR's) and the majority of them use the carrier chrominance signal low-frequency band conversion recording system (M system). In this system, a composite video signal is separated into a luminance signal (hereinafter referred to as a Y signal) and a chrominance signal (hereinafter referred to as a C signal). The Y signal is converted to a corresponding FM signal of the Y signal (hereinafter referred to as FM-Y) by frequency modulation, while the C signal is converted to a low-frequency chrominance signal (hereinafter referred to as C.sub.LOW) by frequency conversion. These FM-Y and C.sub.LOW signals are mixed and recorded on a video tape, and reproduced by the inverse process.
In this M system, the recording and reproduction of the high frequency component of the Y signal suffers various restrictions. Here, FIGS. 12(a)-(g) are used for explanation. FIGS. 12(a)-(g) show a schematic diagram of the video signal spectrum during recording and reproduction. A composite video signal (FIG. 12(a)) including a Y signal band of 4.2 MHz and a C signal carrier of 3.58 MHz is separated into the Y signal (FIG. 12(b)) and the C signal (FIG. 12(c)) by a low-pass filter (hereinafter referred to as an LPF) and a band-pass filter (hereinafter referred to as a BPF). In this case, the frequency band of the Y signal is limited to y.sub.1b from y.sub.1a as indicated by the arrow mark. Thereafter, the Y signal is converted to FM-Y (FIG. 12(d)) by frequency modulation, and the C signal converted to C.sub.LOW (FIG. 12(e)) by low-frequency conversion, and these signals are combined. At this time, the FM-Y signal is sent to the high-pass filter (hereinafter referred to as an HPF) and C.sub.LOW is sent to the LPF and thereafter these are mixed in order to separate the frequencies of FM-Y and C.sub.LOW. From FIGS. 12(a)-(g), it is understood that the band-width of the Y signal is limited again to y.sub.1f when the signal FM-Y passes through the HPF. The recording signal (FIG. 12(f)) thus modulated is recorded on a magnetic tape and thereafter it is reproduced (FIG. 12(g)). It is also obvious that the band-width y.sub.1g of the reproduced Y signal is y.sub.1f in maximum. Namely, the input composite video signal has the band-width y.sub.1a as the Y signal but such a band width is limited to y.sub.1g in maximum when such a signal is recorded and reproduced. In the current home VTR, the recording and playback band-width is about 2 MHz. The band-width of the Y signal can be expanded (corresponding to y.sub.1b of FIG. 12(b)) after separation by using a comb filter for separating the Y signal and C signal but the band-width is restricted by the HPF after frequency modulation and currently the recording and playback band-width is 2.5 MHz in maximum in the frequency allocation of a home VTR.
Meanwhile, methods of obtaining a high quality reproduced picture utilizing a metal tape as the recording medium are being investigated. Namely, recording and playback with a metal tape have remarkably increased playback output and an improved S/N as compared with an existing tape and moreover the carrier frequency in the higher frequency band of FM-Y allows expansion of the recording and playback frequency band of the luminance signal.
Such a high quality VTR is capable of recording and reproducing a signal, for example, of 3 MHz bandwidth.
Meanwhile, the teletext system is now investigated for practical use. In the teletext system, characters or figures are disassembled and these are superposed on the vertical blanking period of the video signal of a TV broadcasting signal and they are selectively received by a home television receiver and displayed on it. For example, the transmitting signal is transmitted, for example, in the format of 5.727272 M-bit/sec in the character transmission. Namely, the signal band-width of 2.86 MHz must be acquired in order to record and reproduce such a signal.
For the home VTR with the carrier frequency of 3.4 MHz.about.4.4 MHz, it has been impossible to record and reproduce the signal of 2.86 MHz but the recording and playback of the teletext signal of 2.86 MHz is considered to be possible because a high quality VTR using a metal tape has a recording and reproducing band-width of 3 MHz or more. However, actually, recording and playback is very difficult because the teletext signal must be reproduced with very good amplitude and group delay characteristics.
However, in the above VTR, a luminance signal passes various filters in the recording and reproduction processes and therefore it has been difficult to make flat the amplitude characteristic of 2.86 MHz and group delay characteristic while the desired cut-off characteristic is maintained. Moreover, a filter having the desired frequency characteristic and flat group delay characteristic is complicated in structure and is expensive and it is not adequate for use in the home VTR because the home VTR also uses many filters. In addition, it is not adequate to only expand the frequency bandwidth of the Y signal because the S/N is deteriorated and noise also increases due to migration of the color signal.
It is also considered difficult to record and reproduce the teletext signal in that a high frequency band is allocated to the luminance signal by separating the video signal into the Y signal and the C signal by the comb filter.
In the above VTR, the teletext signal having no H correlation is reduced by as much as about 6 dB by the comb filter used for separation of the Y and C signals and moreover the group delay characteristic is also disturbed. Thereby, satisfactory recording cannot be done on the tape.
Even in a high quality VTR mentioned above, it is difficult to record the signal of about 3 MHz with flat amplitude characteristic and phase characteristic and particularly the amplitude characteristic is somewhat deteriorated. It can easily be thought that the gain of frequency characteristic in the higher frequency band is raised in order to compensate for deterioration of the amplitude characteristic but it results in a deterioration of the S/N of reproduced video signal.
The Japanese Patent Application No. 57-106909 (Yasuoka et al) discloses a concept that the recording and playback band of the luminance signal is expanded and the character signal is recorded or reproduced by recording or reproducing only the luminance signal without recording and reproducing the color signal in the duration where the character signal is superposed. However, according to this method, a problem arises in that continuity of color signal is lost because the color signal is not recorded and reproduced.